What's the biggest risk factor for the vast majority of diseases?

Sometimes you hear something startling, then you immediately think, "of course, how could it be otherwise?" You knew it all along, but had never heard it said.

That happened to me today. Before you click through to the answer, try to guess it.

I heard someone on the radio, talking about aging. He said: "Aging is the single biggest risk factor for the vast majority of diseases." I had never thought about it that way before, but it makes perfect sense.

He went on to talk about aging itself versus the diseases caused by aging. Which do you think shortens lives more? It turns out that if you got rid of all diseases, it would lengthen our lives moderately, but if you got rid of aging, it would lengthen our lives substantially (we would still die eventually of disease or accident).